Thanks nacho. Yeah, it wasn't really creative but it did have a strong use of voice and was fairly personal. It also had some use of slang as you mentioned. Those don't necessarily make it leave the expository format but it really isn't a traditional expository piece. This was written about 2 months before my exam in preparation for my final SAC.
Expository is simply about 'exposing' a concept. You are examining a certain idea or set of ideas in a mature format such as a newspaper feature column, a personal diary, a blog, etc. You can give your opinion but your main purpose is to explore a topic in a considered and thoughtful manner. Here's an example of a beautiful expository piece:
http://www.theage.com.au/opinion/evil-but-chillingly-ordinary-20090322-95lo.html - By the way, note that it's in the opinion section but it's not exactly a persuasive piece. Although the author does present a point of view, their aim is to offer us an insight - something to ponder - they aren't trying to persuade us forcefully.